1. Field of the Invention
The field of the invention is optical connection transmission networks. The invention relates more particularly to optical networks adapted to relatively limited geographical dimensions, such as metropolitan access networks.
2. Description of the Prior Art
As a general rule, an optical network consists of a plurality of stations able to send and receive optical signals to and from other stations of the network. These exchanges of information are effected by means of optical connections to which are connected access nodes that serve the respective stations.
To exploit the bandwidth capacity of the optical connections, wavelength division multiplexing (WDM) is advantageously used. Accordingly, the optical connections carry multiplex signals formed of a combination of optical signals each consisting of an optical carrier wave modulated as a function of the information to be sent. Each carrier wave has a specific wavelength that defines a corresponding spectral channel.
Moreover, if the network is of sufficiently limited size, providing devices for individual regeneration of the channels may be avoided. Such a network, which is then referred to as “transparent”, may nevertheless include optical amplifiers disposed to amplify simultaneously the channels of the WDM multiplexes transmitted. In the absence of such amplifiers, the network is referred to as “passive”.
Accordingly, the field of the invention is that of transparent networks, passive or otherwise.
A first type of prior art transparent WDM network can use a ring configuration. The network then includes, for example, an optical connection one end whereof is coupled to a send interface of a hub and the other end whereof is coupled to a receive interface of the same hub. The hub is also normally adapted to communicate with an external interconnection network.
FIG. 1 represents one example of such a network diagrammatically in the most simple situation where the looped connection consists of a single fiber F to which are coupled access nodes AN1-AN3 for receiver terminals RX and sender terminals TX of associated stations ST1-ST3.
The optical connection therefore consists of a plurality of fiber sections FS1-FS4 separated by the nodes (and where applicable by optical amplifiers, not shown). The connection has a first end E1, called the upstream end, connected to the send interface HT of the hub HUB and a second end E2, called the downstream end, connected to the receive interface HR of the hub.
The send interface HT is provided with a plurality of senders using carrier waves with different send wavelengths. Also, each station includes at least one receive-wavelength-selective receiver. Accordingly, each sender of the hub can inject into the connection a signal of given wavelength and when that signal reaches an access node via the fiber, that signal may be processed by the associated station if it includes a receiver tuned to that wavelength.
Conversely, the receive interface HR of the hub is provided with a plurality of respective receive-wavelength-selective receivers, and each station includes at least one sender TX of given send wavelength. Accordingly, a sender of a station can inject into the connection, toward the second end E2, a signal of given wavelength and when that signal reaches the receive interface, it can be processed by the hub thanks to one of its receivers sensitive to that wavelength. Given that all the signals propagate in the connection in the same direction, measures must be taken to avoid interference and conflicts at the level of the receivers. For example, a rule may be imposed whereby all the send wavelengths of the hub and of the stations are all different from each other. Time-division multiplexing may also be used for signals sent by a plurality of senders that would be carried by the same wavelength. Each station could include a receiver terminal and a sender terminal respectively consisting of a plurality of receivers and a plurality of senders tuned to a plurality of wavelengths.
Exchanges of information between stations are effected via the hub in the following manner. Each sender station sends the hub a signal carried by one of the receive wavelengths of the hub. That signal contains an address indicative of the destination station. After reception of the signal and its conversion to electrical form by the hub, the destination address is processed by the management means of the network to determine a receive wavelength of the destination station. The signal is then reconverted to optical form by means of a carrier wave having that wavelength and sent as a downlink signal.
All signals on the connection coming from the hub constitute information traffic known as “downlink” traffic and all signals on the connection coming from the stations constitute “uplink” information traffic.
In the embodiment shown diagrammatically in FIG. 1, the nodes ANi (where i is equal to 1, 2 or 3 in the example represented) each consist of a simple 2-to-2 type coupler. Each coupler has a first input connected to the upstream section FSi, a first output connected to the downstream section FSi+1, a second input connected to the senders TX of the associated station and a second output connected to the receivers RX of that station. Hereinafter, the first inputs and outputs are called the first and second “connection points” of the node to the connection and the second inputs and outputs are called first and second “access points” of the station to the connection.
Thus two adjacent sections FSi, FSi+1 are coupled to each other via a first channel of the 2-to-2 coupler, the upstream section FSi is coupled to the receiver terminals RX via a second channel, and the sender terminals TX are coupled to the downstream section FSi+1 via a third channel, these three channels enabling couplings that are not wavelength-selective.
Consequently, each station may insert into the downstream section signals produced by a variable number of senders TX, tuned to any wavelength. Similarly, for reception, each station may process a variable number of signals by providing the same number of photodetectors coupled to the outputs of means for filtering (or demultiplexing) signals received from the upstream section. It is therefore a simple matter to change the capacity of the network by adding senders and photodetectors, without disturbing traffic in transit through the node. Note that a plurality of stations can process the same channel, to enable the broadcasting of the same signal to a plurality of receivers of different stations.
This embodiment provides great flexibility in the choice of send and receive wavelengths.
In a second type of network (not shown) similar to the previous one, the uplink and downlink traffic are contra-directional, i.e. propagate in opposite directions. In this case, the senders and the receivers of the hub are coupled to the same end of the fiber, for example the end E1, and for the downlink traffic each node ANi performs the function of a “1-to-2” coupler from the hub, via the end E1 and the upstream sections, to the downstream section, on the one hand, and to the receiver terminal of the associated station, on the other hand. Conversely, for the upstream traffic of signals going from the sender terminal to the hub, each node ANi performs the function of a “2-to-1” coupler in the opposite direction, i.e. from the senders of the stations to the receivers of the hub, via the same end E1. It may be noted that the “1-to-2” and “2-to-1” coupling functions referred to above may be effected as in the preceding embodiment by means of a 2-to-2 type directional coupler of which only three ports are used, the senders TX and receivers RX of the associated station being connected to the same second access point. It may also be noted that this second type of network no longer has a ring physical topology but instead a tree topology. In this second type of network, the end E2 is not connected to the hub. In fact, all the senders and receivers of the hub are connected to the same end E1.
As in the first embodiment, the nodes provide couplings that are not wavelength-selective. This latter embodiment therefore has the same advantages in terms of the flexibility in the choice of send and receive wavelengths. It further enables the use of common wavelengths for carrying uplink and downlink traffic.
Other considerations must be taken into account in network design, however, in particular the design of transparent and passive networks.
An important aspect in the dimensioning of a network is the “power budget”, i.e. the processing of the minimum permissible optical powers for the photodetectors of the receivers, and consequently the send powers to be provided and/or the maximum possible lengths of the connections between senders and receivers.
In this regard, the last embodiment mentioned has the advantage that it makes it possible to optimize the optical power budget by appropriate dimensioning of the coupling ratios of the various 1-to-2 couplers constituting the nodes of the network. For example, in the case of a network with no in-line amplification, the optimized coupling ratios may be calculated in the following manner.
There is taken as the minimum receive power a common value for all of the receivers of the network, that value being sufficiently high to take into account the spreads of the characteristics of the photodetectors and aging phenomena. Similarly, the senders in the same network are normally chosen to deliver substantially the same power, which also makes it possible to define a nominal send power value common to all the senders of the network.
Given these conditions, for a network having N nodes, for example, there will be chosen N couplers the N coupling ratios whereof have respective values such that the various optical paths via these couplers between the senders and receivers of the hub and the senders and receivers of the terminals have transmission coefficients having substantially the same common value. This is reflected in N equations in which the unknowns are the N coupling ratios and the parameters are the transmission coefficients of the elements constituting the network.
As the uplink and downlink traffic take the same paths, the coupling ratios are optimized for both types of traffic.
On the other hand, this kind of optimization is generally not possible in the case of the first network. In fact, except in special circumstances, if couplers are chosen the coupling ratios whereof are optimized for the downlink traffic, for example, the coupling ratios for the uplink traffic are then fixed, generally at non-optimum values, and vice-versa.
Another important aspect of network design relates to protection against faults or interruption of the connection.
In the case of ring topologies such as that of FIG. 1, prior art solutions are based on the use of a connection consisting of a plurality of optical fibers, which are normally part of the same cable.
One possibility is to provide two fibers respectively dedicated to uplink and downlink traffic, for example. The senders of the hub are then each coupled to the two ends of the fiber dedicated to downlink traffic so that the downlink signals propagate in the fiber in opposite directions. Similarly, the senders of each station are coupled to the fiber dedicated to uplink traffic at two connection points such that the uplink signals propagate in the fiber in opposite directions also.
Means are then provided in each station for routing received couples enabling each receiver to be selectively coupled to one or the other of the coupling points of the station to the fiber dedicated to downlink traffic. Similarly, the hub is provided with means for routing received signals for selectively coupling each of its receivers to one or the other of the two ends of the fiber dedicated to uplink traffic.
Accordingly, if a section of the ring is broken, appropriate switching of the routing means enables the exchanges of signals between the hub and each station to be maintained.
An alternative solution consists in duplicating the network by dedicating one fiber, called the “working” fiber, to the normal mode of operation and the other fiber, called the “protection” fiber, to the protection mode of operation.
There are then provided in the hub and in each station means for routing the signals sent that selectively couple according to the mode of operation each sender to one or the other of the fibers. There are similarly provided means for routing received signals that selectively couple each receiver to one or the other of the fibers.
The two networks advantageously each conform to the second type described hereinabove i.e. use as access nodes optimized 2-to-1 (or 1-to-2) couplers adapted to route bidirectional uplink and downlink traffic.
Given that the working and protection fibers normally belong to the same cable, a break occurring in one section generally leads to unavailability of both fibers in the same section. It will therefore be necessary in each station for the access couplers to the respective fibers to be disposed symmetrically. Thus, in protection mode the uplink and downlink traffic can always propagate between the hub and each station by using fiber sections that are not faulty (provided of course that only one section is faulty).
Although these solutions involving a plurality of fibers are effective in terms of protection, they lead to an overcost that it is desirable to be able to avoid.
To this end, there may be envisaged a protection system applied in the case of a single-fiber connection. The principle is analogous to the preceding situation, except for switching the coupling of the signals sent from one fiber to the other. There has to be used for each station an access node to the same fiber that consists either of a single 2-to-2 coupler or of two 2-to-1 couplers disposed symmetrically.
In the former situation, the coupling structure is that of the network from FIG. 1 with the drawback already indicated: it is generally not possible to dimension the couplers so that they have coupling ratios optimized both for traffic exchanged between each station and the first end E1 of the fiber, on the one hand, and between each station and the second end E2, on the other hand. The same drawback applies in the latter situation.
An object of the invention is to propose a solution enabling the implementation of a network in which are reconciled properties that are incompatible with the prior art networks described hereinabove, i.e. a network provided with a single-fiber connection, in a ring configuration enabling protection in the event of interruption at one point of the connection, and in which to access the fiber the stations use couplers having coupling ratios optimized both in the normal mode of operation and in the protection mode, i.e. both for traffic exchanged between each station and the first end E1 of the fiber, on the one hand, and between each station and the second end E2, on the other hand.